Early development of the chicken lung (Figures 13–17) and expression of basic FGF (bFGF-2) at different stages of development (Figures 18–21). Figures 13 and 14: Lung buds (arrows) on the third- (Figure 13) and fourth days (Figure 14) of embryonic life: initially, the bud comprises of a rather homogenous group of cells (Figure 13) while a little later the epithelial cells (EC) and the mesenchymal cells (MC) reorganize into separate areas (dashed lines). Toluidine blue stained preparations. Scale bars: Figure 13, 100 μm; Figure 14, 50 μm. Figures 15–17: Matrigel cultured lung buds. On the fourth day of embryonic life (Figure 15), a secondary bronchus (dashed circle) can be seen sprouting from a primary bronchus (PB) and at the fifth day of life (Figure 16), extrapulmonary primary bronchi (PB) can be seen continuing into the developing lung and giving rise to secondary bronchi (SB). Tr, trachea; GIT, developing part of the gastrointestinal system. MC, mesenchymal cells. Figure 17: Close-up of developing airways at the sixth day of embryonic life. The epithelium lining the primary bronchus (PB) and the secondary bronchi (SB) can be seen (arrows). Figures 16 and 17 are colour adapted images of black and white microscopic originals. Scale bars: Figure 15, 200 μm; Figure 16, 200 μm; Figure 17, 0.2 mm. Figures 18 and 19: Transverse- (Figure 18) and longitudinal (Figure 19) sections of developing chicken lungs at the sixth day of development showing expression of basic fibroblast growth factor-2 (bFGF-2) in the mesenchymal cells (MC) and in the epithelial cells lining the airways (arrows). SB, secondary bronchi. Scale bar: Figure 19, 100 μm. Figures 20 and 21: Expression of bFGF-2 in the parabronchial epithelial cells (EC) (Figure 20) and in the mesenchymal cells (MC) (Figure 21) of the lung of a seven day old embryo. bFGF is overexpressed in the apical parts of the epithelial cells (arrows) (Figure 20) and in the mesenchymal cells (Figures 20 and 21). PL, parabronchial lumen; stars, basement membrane of epithelial cells; circle, developing blood vessel; EC, endothelial cell. Figure 13 is from Maina ; Figure 13 is from Maina ; other figures-unpublished.